High lift devices for aircraft wings are sometimes referred to as auxiliary airfoils. Such devices or airfoils are extended from the leading or trailing edges of the wing to increase aerodynamic lift during takeoff and landing of the aircraft. When extended from the wing, the high lift devices increase the effective size, curvature, camber and area of the wing, thereby increasing the lift of the wing for slow speed flight. High lift devices extending from the leading edge of the wing are usually known as slats and those extending from the trailing edge of the wing are known as flaps.
Normally, each high lift device is deployed by two separate but coordinated drive mechanisms, one on the inboard side and the other on the outboard side of the high lift device. Should one of these mechanisms be unable to perform its function, a skewing of the high lift device may occur and jamming or loss of the high lift device may result.
The majority of modern airplane high lift drive systems use actuation methods where all flap drive system elements are driven in synchronization. Because of the tight mechanical coupling of the elements, any element losing synchronization with the rest of the system may cause a mechanical failure and, more importantly, the possibility of a skewed flap, i.e., a high lift flight control surface that is in an asymmetrical position. Undetected skewed flaps are a cause for concern due to the possibility of unpredictable airplane handling characteristics. Recent airplane certification requirements have reacted to this concern by mandating the incorporation of flap skew detection systems in new airplane designs. A problem in designing such a system is that there will always be the relatively smaller skewing movements caused by normal structural deflections, dynamics of the aircraft and temperature changes that would not adversely affect airplane handling characteristics but could confuse such a detection system.